gh-133371: Don't optimize LOAD_FAST instructions whose local is killed by DELETE_FAST (#133383)

In certain cases it's possible for locals loaded by `LOAD_FAST` instructions
to be on the stack when the local is killed by `DEL_FAST`. These `LOAD_FAST`
instructions should not be optimized into `LOAD_FAST_BORROW` as the strong
reference in the frame is killed while there is still a reference on the stack.

Author: mpage

Include/internal/pycore_magic_number.h

@@ -276,6 +276,7 @@ Known values:
     Python 3.14a7 3621 (Optimize LOAD_FAST opcodes into LOAD_FAST_BORROW)
     Python 3.14a7 3622 (Store annotations in different class dict keys)
     Python 3.14a7 3623 (Add BUILD_INTERPOLATION & BUILD_TEMPLATE opcodes)
+    Python 3.14b1 3624 (Don't optimize LOAD_FAST when local is killed by DELETE_FAST)
 
     Python 3.15 will start with 3650
 
@@ -288,7 +289,7 @@ PC/launcher.c must also be updated.
 
 */
 
-#define PYC_MAGIC_NUMBER 3623
+#define PYC_MAGIC_NUMBER 3624
 /* This is equivalent to converting PYC_MAGIC_NUMBER to 2 bytes
    (little-endian) and then appending b'\r\n'. */
 #define PYC_MAGIC_NUMBER_TOKEN \

Lib/test/test_peepholer.py

@@ -1,4 +1,5 @@
 import dis
+import gc
 from itertools import combinations, product
 import opcode
 import sys
@@ -2472,6 +2473,13 @@ def test_unoptimized_if_support_killed(self):
         ]
         self.check(insts, insts)
 
+        insts = [
+            ("LOAD_FAST", 0, 1),
+            ("DELETE_FAST", 0, 2),
+            ("POP_TOP", None, 3),
+        ]
+        self.check(insts, insts)
+
     def test_unoptimized_if_aliased(self):
         insts = [
             ("LOAD_FAST", 0, 1),
@@ -2606,6 +2614,22 @@ def test_send(self):
         ]
         self.cfg_optimization_test(insts, expected, consts=[None])
 
+    def test_del_in_finally(self):
+        # This loads `obj` onto the stack, executes `del obj`, then returns the
+        # `obj` from the stack. See gh-133371 for more details.
+        def create_obj():
+            obj = [42]
+            try:
+                return obj
+            finally:
+                del obj
+
+        obj = create_obj()
+        # The crash in the linked issue happens while running GC during
+        # interpreter finalization, so run it here manually.
+        gc.collect()
+        self.assertEqual(obj, [42])
+
 
 
 if __name__ == "__main__":

Python/flowgraph.c

@@ -2795,6 +2795,11 @@ optimize_load_fast(cfg_builder *g)
             assert(opcode != EXTENDED_ARG);
             switch (opcode) {
                 // Opcodes that load and store locals
+                case DELETE_FAST: {
+                    kill_local(instr_flags, &refs, oparg);
+                    break;
+                }
+
                 case LOAD_FAST: {
                     PUSH_REF(i, oparg);
                     break;